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pic: Plywood 8WD Concept
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I like the placement of the motors and the integrated gearbox. It looks clean.
Can I ask why this is specifically a plywood drivetrain? It looks to me like a sheet metal drivetrain. What makes it plywood? And what design decisions were made knowing it would be made of plywood instead of some metal? |
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Anyway, good looking drivetrain! I like the positioning of the wheels, but how are you going to drive the front/back wheels? Is there a sprocket on the other side of the wheel that is hidden in the render? |
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Though that does bring up another thought. Is there any quick and easy way of adjusting the tension of the belts, such as a vexpro versablock and cam setup? Also, another thing I noticed, was that the lightening hole behind the inset wheels happens to line up just right so that debris could be thrown by the wheels right up into your drive-train gearboxes and potentially your electronics. |
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We ran belts this year (from VexPro) in this way and did not have any issues related to over/under tensioning |
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You have the perfect place to mount fans above the cims, while I do not think they are very useful during competition, if you were to make a practice bot that would be running for extended periods they could be very valuable. Just my 2 cents.
(It looks gorgeous by the way, great job!) |
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Some more specs not in the original post:
-Estimated weight of 34.8lbs (Wood frame weighs 8.8lbs) -Geared 5.82:1 for an adjusted top speed of 12.9 ft/s -Wheels, belts and pulleys accessible from bottom of robot -Includes mounts for encoders, battery and main breaker Here's another view with the top plate removed so it's easier to see the structure and power transmission components: ![]() Quote:
IMHO, the main design differences between plywood and sheet metal are the material thickness, the method of attachment and the lack of bends in wood. -The drivetrain is made out of .25" plywood; if it were aluminum, I'd use .060-.090". -Instead of being riveted like sheet metal, the drivetrain will be assembled with finger joints (http://www.instructables.com/id/How-...at-Right-Angl/) and glued. -You can't bend wood like sheet metal so every section at a different angle is a separate piece. The wood frame has 28 parts; with bent sheet metal, far fewer would be needed. Otherwise, I think designing for plywood and sheet metal is quite similar, since both are very strong in the plane of the material but vulnerable to cracking or bending if force is applied from other angles. Quote:
There is not a tensioner for the belts; we had good experiences with center-center distance belts on the AM14U last year and would be happy to use them again. Good catch about the lightening hole; I'll remove that in the next iteration. Quote:
The next thing I'd like to look into is repackaging a ball shifter into a lower and thinner enclosure to add a lower gear to this drivetrain. I'm also worried about the loads on the CIM pinions in the current gearbox. |
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I've attached a picture of our .75" plywood bumpers from when we rebuilt them after MAR champs. As you can see, there's a large crack down through them. Why do you think our bumpers broke and your frame will hold up? ![]() |
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I for one would love to see a sheet-metal version of this design too. It'd be interesting to compare the two. I really like the layout of the whole thing.
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https://picasaweb.google.com/1177698...CADScreenshots https://picasaweb.google.com/1177698...FabAndAssembly |
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Sheet design is a different animal. I enjoy it because it is very challenging and rewarding. The best recommendation that I can give is to start with something simple, and work your way up from there. Maybe try drawing a square sheet frame first, before getting into all the complicated angles that are part of an octagonal frame. I recommend that people think of sheet metal like paper. All the strength is in the bends. Post your design when you have something drawn up, and get feedback on how to improve it. You can run FEA on it if you are adventurous. |
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I would argue that particularly for a drive train, the 1wk turn on sheet parts is more than sufficient. Our approach with the drive train is to send out the parts for fab week 1 while we are continuing to design the super structure. By the time we have the superstructure designed and sent out for fab, the drive train parts are coming back. Having sponsors help us with fabrication lets us focus on design and speeds up the process. |
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If you want to continue with this kind of layout I recommend doing some calculation on the pinion loading. There are different ways of calculating the load on the pinion, the most common is the Lewis formula. The Lewis formula models the gear tooth as a cantilevered beam with the load applied to the tip. To use this formula you need to know the tangential load on the tooth, the diametric pitch, the face width, and the Lewis form factor. The Lewis form factor is based on the number of teeth on the gear, the pressure angle and depth of the tooth. A table of Lewis form factors can easily be found with a google search. If you want to preform this calculation on paper use the fowling formula: bending stress= (Wt*Pd)/(F*Y) where Wt is the tangential load, Pd is the diametric pitch F is the facewidth Y is the Lewis form factor. If you don't like paper and pencils, lucky for you there are many online calculators, this is one of many: http://www.engineersedge.com/calcula...calculator.htm I should note that many of the online calculators, including the one I linked to, solve for the max load a tooth can take while remaining under a specified amount of stress. The formula used for this calculation is the following: Wt=(S*Y*F)/Dp S is the maximum bending stress the tooth will undergo. It's commonly recommended to make S 1/3 of the materials ultimate strength. These equations are intended for gears operating in idea conditions, meaning they are properly lubricated and not experiencing shock. If your gears are running in un-ideal conditions (this is the case for most FRC gearsets) S should be lowered. These formulas will help give you a pretty good idea whether your gear will fail , but Lewis formulas are only the tip of the iceberg when it comes to gear strength calcs. The Lewis formula does not account for rpm, other geometric properties of the gear, and surface wear. If you want to know more equations I recommend you look up the following: Barth velocity factor: This is a modified version of the Lewis formula that accounts for the gears velocity. AGMA bending stress: This is pretty much a much more advanced version of the Lewis formula with more factors. This formula is probably more accurate than needed for your application, but if you find this topic interesting you might enjoy looking it up. Hertzian contact pressure: This calculation models two cylinders in edge contact and will tell you the max normal pressure at the line of contact. This calculation is very important as it will tell you whether or not the surface of the tooth will wear. If you use the Lewis formula and it indicates failure you don't need to calculate contact pressure. But if the Lewis formula indicates that the max bending stress is in range you might want to calculate the contact pressure to determine if the teeth will significantly wear. -Adrian Edit: I should mention that while the internet is a decent resource for finding and understanding these formulas a machine design book will give you much more information. It wouldn't be a bad idea to talk to one of your ME mentors on the subject, if they can't help you with the formulas they probably have a textbook that can. |
Re: pic: Plywood 8WD Concept
One thing to keep in mind is that the wheels don't have positive traction. So to calculate the force in the pinion gears you need to figure out the loading the wheels will apply before slipping. Adding up the stall torque of the motors and factoring in the gear reduction to the force applied on the pinion tooth is the front half of it. Traction and slippage is the back half. The pinions do take more loading than the usual frc gearbox.
I like the cim layout and have been playing with it around a year. I haven't been satisfied with the packaging to move forward with testing. Its neat to see how others approach similar problems with their designs. 2337 the enginerds have done a similar 8wd sheet metal layout with an integrated two cims and two speed gear box the past two years. I really like what they did. They have their 2013 cad online, you should check it out for design solutions implemented if your interested in this 8wd layout. My concern would be the impact resistance of the 1/4 inch plywood. Metal bends before cracking better than wood in impact situations. |
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I can't see exactly what you're doing with your pinions, but 254's been running double loaded pinions in all their 3 CIM gearboxes.
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Theoretically you can triple load a pinion gear as the drivetrain rocks on one set of wheels or the other as a worse case scenario. If both sides of the gearbox are loaded evenly, its more of a double loaded worse case scenario.
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-Adrian |
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Wow, what responses for a summer post.
I was only thinking what wonderful designs two heads can come up with... |
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-Curious Student |
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3 motors, as the last pinion transfers the torque of the other 2 motors. That works out to 6x total, and really points out the danger of passing load down pinions. |
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I would expect to see very little torque on a motor pinion if the robot is going full speed and full voltage in the opposite direction was applied. Although, I've been wrong many times before... |
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I'll break it down a little more thoroughly. Regardless of how it got that way, if a motor is rotating at some speed, it will generate a voltage (the back EMF). So lets say you were going full speed one way, and then decide to go full voltage the other way. Right before the switch, the Back EMF opposes the battery voltage and you get a very small voltage differential to drive current (which is equivalent to the current required to generate the torque required to match the friction). Let's say the battery is at 12V, and the back EMF is 11.8 (made up numbers), some current calculated by V=IR would flow. When you switch the polarity, the 12V and 11.8V no longer oppose each other, you now have 23.8V across the motor. You once again find I from V=IR, and find that this I is ~ double the I that would be calculated at usual stall conditions. Since Torque is proportional to current, you will therefore see about double stall torque. Another way to visualize this is to extend the classic torque/speed graph to include negative free speed, you'll see the torque there is 2x the stall torque. Extending it past full speed positive shows that you need to apply external torque to get a motor going faster than free speed as well. An important takeaway from all this is most FRC systems are capable of seeing 2x stall torque if they reach full speed one way, and negative full voltage is applied. |
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I would like to crunch some numbers here to see what kind of loading the cim pinions usually take, i have not looked at the numbers before to see where they stand. It seems to me that the simple and conservative way to approach this is to use the stall torque at a steady state, but there are some dynamics involved here. Now a question would be how much of an impact do the system dynamics play into the tooth loading? When the motors reverse is there a ramp or instant loading? Do the wheels slip? I think if those parameters can be modeled, an accurate calculation can be made regarding the loading.
Does that logic make sense or am I missing something? |
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I agree with Adam's analysis with one minor quibble: with 2x stall current you may not get 2x stall torque. Depending on the motor design, the magnetics may saturate before that. Does anyone have stall torque test data for CIM at 24 volts? |
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Also worth pointing out that in especially regards to drive with so many paired motors, you will have 4 or more motors trying to draw near double stall current, over 1000 Amps for 4 cims. This will cause a large voltage drop, until the current draw reaches an equilibrium with that. This will greatly reduce the Torque. My hunch is you will still experience greater than 1x Stall torque at nominal voltage, but not the classical 2x I originally stated. |
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